Fouling as a cause of drift in pressure sensors

by STS Sensors on July 17, 2017
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Fouling as a cause of drift in pressure sensors 

We all know the saying that ‘you can’t make an omelet without breaking eggs’. In the development of new combustion engines specifically, this means that soot particles or oil residue can contaminate the sensors employed.

The consequence of such soiling amounts to increasingly inaccurate readings. When, for example, the exhaust system of a new combustion engine is being monitored with pressure transducers, more and more fine dust will settle on the diaphragm of the sensor over time. The membranes of piezoresistive pressure sensors are very thin so that they can deliver high-precision measurement results. But when a layer of soot forms on this over time, it reduces the overall sensitivity of the pressure transducer.

Protecting pressure transmitters from particulate matter

End users make note of this drift in the sensor by performing reference pressure measurements. They will find considerable differences between the values of this reference pressure gauge and the soiled sensor itself. Often, however, the readings experienced by users reveal when the measured signals deviate too far from the expected results. Strong fluctuations in these measured values can also be an indicator of contamination.

STS generally recommends that users whose sensors are exposed to dirt should service them after a maximum of 100 operating hours. In addition, users can also try to protect the sensor as much as possible from contamination. There are two common methods used here.

Method 1: Protective foil

The first method does not replace maintenance of the sensor after a maximum of 100 hours, but it does simplify cleaning and also preserves the membrane. In this case, a very thin, metallic protective foil is applied to the membrane to protect it from soiling. After a maximum of 100 operating hours, this film is then simply peeled off and replaced with a new one.

Method 2: Cooling adapter

This method allows users to kill two birds with one stone. By screwing a cooling adapter or a climatic valve to the front end of the pressure port, the membrane is now largely protected from soiling. The climatic valve opens only when there is actually something to be measured. Where no permanent pressure monitoring is required, this can be a good method for minimizing the degree of contamination to the sensor employed.

At the same time, a constant sensor temperature can also be ensured via this cooling element. Besides membrane contamination, temperature also has an effect on the measuring accuracy of piezoresistive pressure transducers (More on the influence of temperature on the accuracy of pressure sensors can be found here).

Cleaning of pressure sensors from oil contamination

Contamination with heavy oil particularly comes about in the development of marine engines. The additives incorporated become especially deposited on the membrane and can even damage it. These residues reduce the sensitivity of the pressure transducer and regular servicing must be observed here also.

To keep soiling and the consequences thereof as minimal as possible, consideration should be given to the nature of the pressure sensor at its selection. A stainless steel membrane is recommended, which is front-flush and has no channels in which even the smallest of deposits can gather. The smoother the better also applies here, because on a rough membrane undesired particles will actually deposit faster and these are also more difficult to clean.

To clean a soiled pressure sensor, it must be removed from its application. Isopropanol (IPA) is recommended as the cleaning agent here. While the sensor housing requires no special caution, the membrane should be treated without any firm pressures by using, for example, cotton swabs. Under no circumstances should compressed air be used, since the membranes are very thin and, when too much pressure is exerted, deformations can occur.

Topics: Pressure Transmitter, Pressure Transducer, Pressure Measurement, Piezoresistive Pressure Transmitter, Accuracy, Soiling, Drift

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